(Not applicable)
(Not applicable)
The present invention relates to meters for measuring electrical parameters such as current and in particular to multi-meters which include a clamp-type ammeter.
Meters for measuring current, voltage and resistance or to detect electrical continuity of an electrical device or line are well known. Such meters typically include sensing circuitry as known in the art to measure only one or more of these parameters. Multi-meters are capable of measuring multiple parameters and include a selector for switching the sensing circuitry to detect the desired parameter.
Some multi-meters have a conductive wire coil or metallic loop that can be positioned about an electrical line, cable or bus bar to measure the current passing therethrough by picking up the surrounding inductive field. A closed loop is necessary to provide a closed electrical path to the sensing circuitry of the meter. Some meters, known as clamp-type ammeters, have a split loop disposed in a C-shaped clamp that can be opened to fit around the line. When the clamp is closed the loop again defines a closed path.
The coil or conductive loop is connected in the sensing circuitry so as to only measure current. That is, the loop is inoperative for measuring voltage or resistance. The clamp is only meant to be used when the selector is in the current measurement position. Thus, if a user attempts to measure voltage, for example, with the loop, the multi-meter would not provide a measurement. The same is the case if the user intended to measure current with the loop but the selector was positioned in a voltage or resistance measurement mode. In either situation, this may be frustrating for the user and cause him or her to incorrectly believe that the multi-meter was broken or functioning improperly. This can harm the reputation of the manufacturer and result in returned merchandise or exercised warranties despite the fact that the meter operates properly. This process can be time consuming and expensive for both the user and the manufacturer.
Accordingly, a need exists in for an improved clamp-type multi-meter.
The present invention provides a hand-held clamp-type multi-meter in which the current clamp can be opened only when the meter is set to measure current.
Specifically, the present invention is a multi-meter at least capable of detecting the current and one other electrical characteristic of an electrical line connected to an electrical circuit. The multi-meter has an indicator coupled to sensing circuitry that includes an electrically conductive loop disposed in two jaws of a current clamp. The jaws are opposable so that the current clamp can be separated and fit around the line without disconnecting it from the circuit. The current clamp can be opened only when the multi-meter is set in the current measuring mode.
Thus, the present invention ensures that the conductive loop is used only for measuring current. The multi-meter includes a locking mechanism which prevents the current clamp from opening if the multi-meter is not set to measure current. When the current clamp is open, the locking mechanism also prevents the multi-meter from being set to measure another parameter, such as voltage or resistance.
In a preferred form, the multi-meter includes a palm-sized housing in part containing the sensing circuitry and to which is mounted a measurement selector for switching the sensing circuitry between voltage, current and other parameters. Rotation of the measurement selector to measure current operates the locking mechanism to unlock the current clamp. A thumb lever is connected integrally to a pivotable jaw of the current clamp that is biased to close the jaws. When the multi-meter is set to measure current, the locking mechanism is positioned to allow the thumb lever to be depressed inwardly, which rotates the pivotable jaw away from a fixed jaw and separates the loop. Preferably, both jaws are arcuate members defining a closed path with an opening therebetween. The jaws, which include the loop, may also include an electrically inert covering material, such as plastic, which houses the inductive pick-up loop. The mating ends of the fixed and pivotable jaws of the current clamp have openings that allow the loop to define a closed circuit when the current clamp is closed.